Radio resource control connection resume method of wireless communication system

ABSTRACT

An RRC connection resume method of a wireless communication system is provided. An RRC suspend message is received by a UE from a first base station. An RRC resume procedure is performed by the UE with a second base station in response to the RRC suspend message. An RRC resume response is received by the UE from the second base station.

CROSS REFERENCE

This application claims the benefit of U.S. Provisional Application Ser.No. 62/451,937, filed on Jan. 30, 2017, and entitled“INTRA-RAT/INTER-RAT RRC RESUME PROCEDURES FOR MOBILITY ENHANCEMENT”,which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to radio resource control (RRC)connection resume methods, and more particularly, to RRC resumeprocedures for a wireless communication system related to a Narrow-BandInternet of Things (NB-IoT).

BACKGROUND

The next generation new radio Narrow-Band Internet of Things (NR NB-IoT)is a new radio access technology of the next generation radio accessnetwork (RAN) that the 3^(rd) Generation Partnership Project (3GPP) isdeveloping to support a large amount of low-cost device (e.g., 10⁶devices/Km²) to exchange (e.g., transmit and receive) data in the nextgeneration radio access network (e.g., 5G-RAN). Based on the legacyNB-IoT, which is the NB-IoT specifications designed in 3GPP Release 13and Release 14 specifications for Internet of Things, the NR NB-IoT isto integrate with the legacy NB-IoT to co-exist with other Radio AccessTechnologies (RATs) of the next generation RAN, such as the nextgeneration wide band (e.g., 5G-WB) technology. As the next generationwide band (e.g., 5G-WB) RAT uses millimeter wave transmission andreception, which suffer from high path loss, the NR NB-IoT may be usedto cover areas within the next generation radio access network (e.g.,5G-RAN) that are not covered by the 5G-WB. That is, the NR NB-IoT maysupplement or cover the so-called “wide band holes” within the nextgeneration radio access network (e.g., 5G-RAN). For example, since thenext generation wide band (e.g., 5G-WB) cells are relative small (e.g.,with a coverage radius of 100 meters), if a user equipment (UE) movesout of the next generation wide band cells and still keeps looking forthe wide band cells, the UE may have to frequently conduct cellselection and reselection. The frequent cell selection and reselectionare both power inefficient for the UE and a waste of the radio accessresources of the wide band cells. Moreover, the NR NB-IoT is to co-existwith legacy Global System for Mobile Communications (GSM), GeneralPacket Radio Service (GPRS), and Evolved Universal Terrestrial RadioAccess (E-UTRA) networks. Thus, there is a need in the art for methodsto handle NR NB-IoT related intra-RAT and inter-RAT procedures. In thisdisclosure, the intra-RAT and inter-RAT is differentiated based on5G-RAT, which covers 5G-WB technology and NR NB-IoT technology. A UEswitching between 5G-WB cell and NR NB-IoT cell is regarded as anintra-RAT procedure, while switching between a 5G-RAT with other RATs(e.g., E-UTRA) is regarded as an inter-RAT procedure.

In addition, since a NR NB-IoT cell may cover a relatively large area(e.g., with a coverage radius of several kilometers) as compared to anext generation wide band cell (e.g., a 5G-WB cell), the NR NB-IoT needsto provide different coverage enhancement (CE) levels/labels based onthe signal strength seen by the UEs in the NR NB-IoT cell. For example,the CE levels/labels may indicate how many times the UE or its servingbase station needs to repeat the transmission to mitigate theinefficiency of the control and/or data channel. For example, accordingto the 3GPP Release 13, in legacy NB-IoT, a UE needs to conduct a signalstrength measurement by itself to estimate an appropriate CE level/labelto decide on, among other things, the CE label's range, the associatedUE resource for transmission, and how many times the UE needs to repeatthe control and/or data signal. However, the CE level/label estimated bythe UE may not be sufficiently accurate to ensure a successfultransmission from the UE to the legacy NB-IoT. This is because theuplink channel condition as seen by the legacy NB-IoT base station maybe worse than the downlink channel condition as seen by the UE, which isreflected in the CE level/label based on the UE's measurement. Thus, inlight of introduction of the NR NB-IoT in the next generation radioaccess network (RAN), there is a need in the art for methods to provideaccurate coverage enhancement levels/labels to handle NR NB-IoT relatedintra-RAT and inter-RAT procedures.

Furthermore, in a next generation radio access network (e.g., 5G-RAN), aUE may transition from an RRC connected state to an RRC inactive stateor an RRC idle state, where the UE may take measurements to perform cellselection and cell reselection, even when the UE is in the RRC idle orRRC inactive state. For the RRC state transitions, an RRC suspendmessage and an RRC Resume message may be used. For example, in a nextgeneration radio access network (e.g., 5G-RAN), the RRC Suspend messagemay inform a UE that the serving base station (e.g., a next generationnode B (gNB)) will release the RRC connection, but will retain the UEcontext. When there is a need for packet exchange, the UE can quicklysend a RRC Resume message, so that the UE and serving base station canresume an RRC connection for data/control signal transmission to reducelatency and radio access overhead.

Thus, in light of introduction of the NR NB-IoT in the next generationradio access network (RAN), there is a need in the art for methods tohandle NR NB-IoT related intra-RAT and inter-RAT RRC resume proceduresfor mobility enhancement.

SUMMARY

In one aspect of the present disclosure, an RRC connection resume methodof a wireless communication system is provided. An RRC suspend messageis received by a UE from a first base station. An RRC resume procedureis performed by the UE with a second base station in response to the RRCsuspend message. An RRC resume response is received by the UE from thesecond base station.

In another aspect of the present disclosure, a UE is provided. The UEincludes a processor configured to perform the followings instructions.An RRC suspend message is received from a first base station. An RRCresume procedure is performed with a second base station in response tothe RRC suspend message. An RRC resume response is received from thesecond base station.

In yet another aspect of the present disclosure, a base station isprovided. The base station includes a processor configured to performthe followings instructions. An RRC suspend message is transmitted tothe UE. A UE context fetch request is received from a second basestation. A UE context fetch response is transmitted to the second basestation in response to the UE context fetch request.

In yet another aspect of the present disclosure, a base station isprovided. The base station includes a processor configured to performthe followings instructions. An RRC resume request is received from theUE. A UE context fetch request is transmitted to a first base station inresponse to the RRC resume request. A UE context fetch response isreceived from the first base station. An RRC resume response istransmitted to the UE in response to the UE context fetch response.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating various legacy NB-IoT deploymentoptions in a legacy long-term evolution (LTE) network.

FIG. 2 is a flowchart of an RRC connection resume method of a wirelesscommunication system, according to an exemplary embodiment of thepresent disclosure.

FIG. 3 is a diagram illustrating an intra-RAT RRC resume procedure of awireless communication system for packet transmission and/or reception,according to an exemplary implementation of the present application.

FIG. 4 is a diagram illustrating an RRC resume procedure achieved by a2-step radio access procedure, according to an exemplary implementationof the present application.

FIG. 5 is a diagram illustrating an RRC resume procedure achieved by a4-step radio access procedure, according to an exemplary implementationof the present application.

FIG. 6 is a diagram illustrating an intra-RAT RRC resume procedure forradio access network (RAN)-based notification area (RNA) update,according to an exemplary implementation of the present application.

FIG. 7 is a diagram illustrating an intra-RAT RRC resume procedure forcore network (CN)-based tracking area update, according to an exemplaryimplementation of the present application.

FIG. 8 is a diagram illustrating an inter-RAT RRC resume procedure forpacket transmission and/or reception, according to an exemplaryimplementation of the present application.

FIG. 9 is a diagram illustrating a validity period for UE contextstorage at UE and source cell for inter-RAT/intra-RAT RRC resumeprocedure, according to an exemplary implementation of the presentapplication.

DETAILED DESCRIPTION

The following description contains specific information pertaining toexemplary embodiments in the present disclosure. The drawings in thepresent disclosure and their accompanying detailed description aredirected to merely exemplary embodiments. However, the presentdisclosure is not limited to merely these exemplary embodiments. Othervariations and embodiments of the present disclosure will occur to thoseskilled in the art. Unless noted otherwise, like or correspondingelements among the figures may be indicated by like or correspondingreference numerals. Moreover, the drawings and illustrations in thepresent disclosure are generally not to scale, and are not intended tocorrespond to actual relative dimensions.

FIG. 1 is a diagram illustrating various legacy NB-IoT deploymentoptions in a legacy long-term evolution (LTE) network. In FIG. 1, thelegacy NB-IoT may have three deployment options, namely, in-banddeployment, guard-band deployment, and stand-alone deployment. In FIG.1, the legacy NB-IoT may occupy 180 KHz (i.e., 180×10{circumflex over( )}3 Hertz) minimum system bandwidth for both downlink (DL) and uplink(UL) operations. The in-band deployment of the legacy NB-IoT may makeuse of the same resource block in a long-term evolution (LTE) carrier,such as a 10 MHz (i.e., 10×10{circumflex over ( )}6 Hertz) LTE carrier,of a legacy LTE network. The guard-band deployment of the legacy NB-IoTmay be done using the guard-band of the existing LTE network. Thestand-alone deployment of the legacy NB-IoT may use a bandwidth outsideof the LTE carrier bandwidth of the existing LTE network.

According to an exemplary implementation of the present implementation,the NR NB-IoT may have the same deployment options as the legacy NB-IoT.In addition, the NR NB-IoT can be integrated into the next generationwireless communication networks, such as a next generation radio accessnetwork (e.g., 5G-RAN). For example, the NR NB-IoT deployment in FIG. 1may be applied to the next generation radio access network (e.g.,5G-RAN) by replacing the LTE cell with a next generation wide-band(e.g., 5G-WB) cell. It is noted that “wide-band” is to used here todifferentiate the next generation new radio cell (e.g., a 5G-NR cell)from a NR NB-IoT cell, which operates on, for example, 180 KHzbandwidth. The next generation wide-band cell may have carrierfrequencies ranging from 6 GHz to more than 60 GHz, with wide bandwidthranging from millimeter-wave to micro-wave bands. However, it is alsoworthy to note that NR wide-band operation under 6 GHz is not excluded.In the next generation radio access network (e.g., a 5G-RAN), a basestation, such as a next generation node B (gNB), may provide both thenext generation wide-band (e.g., 5G-WB) cell(s) and NR NB-IoT cell(s).Different NR NB-IoT cell(s) and 5G-WB cell(s) within a gNB may beregarded as different cells. The gNB can control UEs to switch to a NRNB-IoT cell or a 5G-WB cell, both belong to a next generation radioaccess network (e.g., 5G-RAN). In some embodiments, a NR NB-IoT cell mayshare one physical cell ID with at least one 5G-WB cell. Under thiscondition, the NR NB-IoT cell is treated as one physical layerconfiguration in the 5G-RAN.

A NR NB-IoT cell may provide a large service coverage for a number ofUEs. A UE may connect to the NR NB-IoT cell based on a coverageenhancement level (CE level) configuration to improve radio resourceallocation efficiency. In one implementation, a NR NB-IoT cell mayconfigure multiple CE levels, each CE level having an individual radioresource configuration. For example, the radio resource configurationmay cover the following resource domains:

-   -   (1) Time domain: periodicity of radio resource.    -   (2) Frequency domain: frequency location (in terms of subcarrier        offset), and number of subcarriers.    -   (3) Code domain: Preamble (group) configuration for random        access.    -   (4) Number of repetitions in packets transmission when the UE is        transmitting/receiving data and control signaling with the NR        NB-IoT cell.    -   (5) Resource allocations for UE to exchange data/control        signaling with the NR NB-IoT cell. Also note the resource may be        configured as periodic or aperiodic in time domain.

A UE in the NR NB-IoT cell may take the UE's downlink (DL) measurementsof the NR NB-IoT cell to obtain received signal strength (e.g.,Reference Signal Received Power (RSRP), Reference Signal ReceivedQuality (RSRQ), Received Signal Strength Indicator (RSSI), or SINR(Signal to Interference plus Noise Ratio)) to decide the CE level basedon the DL measurement result. The UE may then select the radio resourceand random access preamble transmission power based on the estimated CElevel for the random access process. It is worthy to note that a randomaccess failure may occur to the UE. After the random access failurehappens for several times, the UE may reselect another CE level andundergo the random access process again to redo random access with thegNB.

In the next generation radio access network (e.g., 5G-RAN), a nextgeneration base station (e.g., a gNB) may suspend RRC connection andradio bearers (e.g., Signal Radio Bearer, Data Radio Bearer) of a UE byusing an RRC suspend procedure. To suspend the RRC connection, theserving base station (e.g., a gNB) of the UE may send an RRC connectionrelease message with resume identity to the UE. Then, the UE may storethe UE's context, which include AS context and security context, andthen suspend the RRC connection after receiving the RRC connectionrelease message. The serving base station (e.g., a gNB) may also storethe UE's context when the UE's RRC connection is suspended.

After the RRC connection is suspended, the UE may request to resume thesuspended RRC connection by sending RRC connection resume request withthe given resume identity to a target base station, which may be theoriginal serving base station sending the RRC suspend message to the UE(e.g., a gNB or an eNB), or a different base station in the RAN (e.g., agNB or an eNB). In response to the request to resume the RRC connection,the target base station may (1) resume the suspended RRC connection ofthe UE; or (2) reject the request to resume and instruct the UE toeither keep or modify (part of) the stored context; or (3) setup a newRRC connection for the UE. It is also worthy to note the target basestation (e.g., a gNB or an eNB) and the serving base station (e.g., agNB) may be different base stations. So, the target base station mayrequire UE's context from the serving base station through a processcalled UE context fetch.

FIG. 2 is a flowchart of an RRC connection resume method of a wirelesscommunication system, according to an exemplary embodiment of thepresent disclosure. In step S210, an RRC suspend message is received bya UE from a first base station. In step S220, an RRC resume procedure isperformed by the UE with a second base station in response to the RRCsuspend message. In step S230, an RRC resume response is received by theUE from the second base station.

In one embodiment, the first base station and the second base station isrealized by different radio access numerologies (e.g., sub-carrierspacing, cyclic pre-fix length, DL/UL configuration, frame structure),where one of the base stations is wide-band numerology and another oneis narrow-band-Internet-of-Things numerology. In another embodiment, thefirst base station and the second base station is realized by differentradio access technologies, where one of the base stations isnarrow-band-Internet-of-Things numerology.

In one embodiment, the RRC suspend message includes an RRC resume UE IDdelivered by the source cell to the UE. In some embodiments, the RRCresume UE ID includes a source cell ID (or truncated source cell ID).

In one embodiment, the RRC suspend message includes an RRC suspend type.For example, the RRC suspend type may include an inter-RAT mobility oran intra-RAT mobility. The source cell may indicate the RRC suspend typein the RRC resume message for the UE. Thus, the UE could know how todecode the RRC suspend message correctly by obtaining the RRC suspendtype.

In one embodiment, the RRC suspend message includes target radio accesstechnology (RAT) information, e.g. a target PLMN (public land mobilenetwork) ID. For example, the source cell may indicate the target RATfor inter-RAT mobility support when the RRC suspend type is theinter-RAT mobility so that the UE could select the target cell. In oneembodiment, the target radio access technology information includes abandwidth part configuration, such as a bandwidth configuration infrequency domain, numerology information, or coverage enhancement levelinformation. For example, the coverage enhancement level information isan authorized CE level(s) the UE is authorized to apply with a legacyNB-IoT (or a NR NB-IoT cell). The authorized CE level(s) of the UE isprovided by the upper layers of RAN. In another embodiment, the coverageenhancement level information is a suggested CE level of the targetlegacy NB-IoT cell (or a NR NB-IoT cell) suggested by the source cellfor UE. The source cell may decide the suggested CE level based on UE'smeasurement reports to the source cell. In some embodiments, the sourcecell may also measure the channel qualities (e.g., RSRP, RSRQ, RSSI,SINR) of UE's uplink data or control channels to decide the suggested CElevel for UE. The coverage enhancement level information includesresource configuration in time/frequency/code domains and number ofrepetitions for DL/UL packet transmissions.

In one embodiment, the RRC suspend message includes target cellinformation. For example, the target cell information may includesupport information to help the UE to find out at least one target cell,such as a target cell ID, e.g., a physical cell ID of the targetcell(s), which may be broadcasted by the target cell periodically. Thesupport information may include a frequency band deployment (e.g.,In-band/guard-band/standalone deployment) of the NB-IoT target cell(legacy NB-IoT cell or a NR NB-IoT cell). The support information mayinclude a frequency carrier deployment, e.g., ARFCN (absoluteradio-frequency channel number), EARFCN (E-UTRA absolute radio-frequencychannel number), PRB (Physical resource block) index. The supportinformation may include an operating bandwidth, e.g., 180 KHz for alegacy NB-IoT cell or a NR NB-IoT cell.

In one embodiment, the RRC suspend message includes an area information.For example, the area information includes tracking area information ora radio access network (RAN)-based notification area (RNA)configuration. The tracking area information (TAT) may include at leastone physical cell ID or at least one tracking area ID for UE to performcore network-based tracking area update procedures. The TAI would berepresented by at least one physical cell ID or at least one trackingarea ID. The RNA configuration may include at least one physical cell IDor at least one RNA ID for UE to perform RNA update procedures.

In one embodiment, the RRC suspend message includes an RRC resumevalidity timer. The RRC resume validity timer may be utilized toinvalidate UE context for UE side. UE may remove at least a part of UEcontext when the RRC resume validity timer expires.

In one embodiment, the RRC suspend message may include a definition ofUL small packet transmission. For example, the definition of smallpacket (e.g. upper bound of the size of small packet) may be configured.Then, UE may deliver small packet transmission with the RRC resumerequest if the size of pending packet in the UE's buffer is smaller thanthe pre-defined value defined in the RRC suspend message. In some otherembodiments, the definition of UL small packet transmission may beprovided through broadcast message from the serving base station ortarget base station. When the definition of UL small packet isbroadcasted by the target base station, UE would apply the definition ofUL small packet broadcasted by the target base station. In some otherembodiments, UE would apply the UL small packet definition broadcastedby the source base station. Also note that the UL small packetdefinition obtained by the RRC suspend message would have higherpriority. Therefore, UE would not apply the UL small packet definitionbroadcasted by the source base station or target base station if the UEhas a valid UL small packet definition configured by RRC suspendmessage.

In one embodiment, the RRC resume response indicates that whether theRRC resume procedure is performed successfully or not. In someembodiments, the RRC resume response further includes data packetacknowledgement (ACK) or Non-acknowledgement (NACK). For example, whenthe small uplink packet is transmitted with the RRC resume procedure,the second base station may transmit an ACK to the UE after the smalluplink packet is received successfully, or transmit a NACK to the UE ifthe small uplink packet is not received.

In some embodiments, the RRC resume response further includes an areaupdate response, when the RRC resume procedure includes an area updaterequest (e.g., an RNA update or a CN-based tracking area).

FIG. 3 is a diagram illustrating an intra-RAT RRC resume procedure of awireless communication system 300 for packet transmission and/orreception according to an exemplary implementation of the presentapplication. The wireless communication system 300 includes UE 302,source cell 304 (serving base station), and target cell 306 (target basestation). An intra-RAT RRC resume procedure enables a UE to switch froma source cell to a target cell both in a next generation radio accessnetwork (e.g., 5G-RAN). For example, the UE may switch from a sourcecell (e.g., a next generation wide-band cell, such as a 5G-WB cell) to atarget cell (e.g., a NR NB-IoT cell).

In this embodiment, source cell 304 is a next generation wide band cell,while target cell 306 is a NR NB-IoT cell, where source cell 304 andtarget cell 306 are implemented under the same radio access technologybut under different numerologies. In some embodiments, both the sourcecell and the target cell are 5G-WB cells or NR NB-IoT cells. In someembodiments, the source cell is a NR NB-IoT cell and the target cell isa 5G-WB cell.

In step S310, UE 302 may receive an RRC suspend message from source cell304 (e.g., a 5G-WB cell), for example, when UE 302 or source cell 304determines that the radio link quality between them is getting poorand/or satisfies one or more predefined conditions. For example, onepredefined condition may be a reference signal measurement result(s)being lower than a threshold value(s). In this case, the RRC suspendmessage may include an RRC resume UE ID. Source cell 304 may configureUE 302 to transition from RRC connected state to RRC inactive state (orRRC idle state). In addition, source cell 304 may provide supportinginformation (e.g., target RAT information, or target cell information)in the RRC suspend message to help UE 302 find a target cell. In someimplementations, source cell 304 may provide the supporting informationbased on UE 302's measurement report about the signaling qualities(e.g., RSRP, RSRQ, RSSI, SINR) which UE 302 has measured from neighborcell(s). Under this condition, source cell 304 may require UE 302 tosend its measurement report before source cell 304 provides thesupporting information.

In step S320, UE 302 may find target cell 306 (e.g., a NR NB-IoT cell),and implement RRC resume procedure with target cell 306. UE 302 mayinclude the RRC resume UE ID in the RRC resume procedure to target cell306.

In step S322, after receiving the RRC resume procedure from UE in stepS320, target cell 306 may attempt to obtain the UE context of UE 302from source cell 304 by sending a UE context fetch request to sourcecell 304. There are several approaches for target cell 306 to identifysource cell 304. In one approach, target cell 306 may identify sourcecell 304 by using the source cell ID included in the RRC resume UE ID.In another approach, for target cell 306 to identify source cell 304, UE302 may add the source cell ID, which is broadcasted by source cell 304(e.g. in the system information of source cell 304), in the RRC resumeprocedure. In step S324, source cell 304 may send a UE context fetchresponse to target cell 306. The UE context fetch response may includethe UE context of UE 302.

In step S330, target cell 306 may send an RRC resume response to UE 302when target cell 306 obtains the UE context of UE 302 from source cell304. In this case, the RRC resume response is a success message when theUE context of UE 302 is successfully obtained from source cell 304 bytarget cell 306.

In step S332, UE 302 may send an RRC resume complete message to targetcell 306. In step S334, after receiving the RRC resume complete messagefrom UE 302, target cell 306 may send a UE context release message tosource cell 304 to let source cell 304 release or remove the UE contextof UE 302. In step S336, source cell 304 may optionally forward the UEpackets of UE 302, which are buffered in the source cell 304, to targetcell 306.

After the RRC resume complete message, UE 302 may transition from RRCinactive state (or RRC idle state) to RRC connected state, which isbased on the instruction of the target cell 306 in the RRC resumeresponse message S330. In step S340, UE 302 may establish a dataconnection for packet transmission and/or reception with target cell306. Thus, UE 302 may continue data exchange even when it enters thewide band holes of 5G-WB cells.

It should be noted that, in some embodiments, target cell 306 mayrequire UE 302 to rebuild a new RRC connection by sending RRC resumeresponse (new RRC connection) to UE 302. Then, UE 302 may clear thestored UE context and then start RRC connection establishment processwith target cell 306.

It should also be noted that, in some other embodiments, target cell 306may reject the RRC resume procedure by sending RRC resume response witha rejection message to UE 202, for example, when target cell 306 couldnot obtain the UE context from source cell 304. When UE 302 is rejectedby target cell 306, UE 302 may remove target cell 306 from the suitablecell(s) and find another cell for RRC resume procedure.

It is also worthy to note that, in some implementations, target cell 306may not have direct backhaul connection with source cell 304. In suchcase, target cell 306 may deliver UE context fetch request to a corenetwork (not explicitly shown in FIG. 3). The core network may contactwith source cell 304 for UE context fetch request and then forward theUE context to target cell 306.

In one embodiment, after receiving the RRC suspend message, UE 302 maystart packet reception if UE 302 receives paging message from the nextgeneration radio access network (e.g., 5G-RAN). Then, UE 302 may alsostart the random access procedure and the following RRC resumeprocedure.

The RRC resume procedure may include a 2-step radio access procedure asdescribed with reference to FIG. 4, or 4-step radio access procedure asdescribed with reference to FIG. 5.

FIG. 4 is a diagram illustrating an RRC resume procedure achieved by a2-step radio access procedure, according to an exemplary implementationof the present application. The wireless communication system 400includes UE 402 and target cell 406.

In step S410, UE 402 may multiplex a preamble sequence in MSG 1 with anRRC resume request in MSG 1, where the preamble may be given by RAN inthe RRC suspend message or be given by UE autonomous selection from apre-defined preamble group. Target cell 406 may receive the RRC resumerequest in the MSG1 of random access procedure. In some embodiments, forUL small packet transmission, the small data may be multiplexed with thepreamble and the RRC resume request in the MSG 1. In step S420, targetcell 406 may deliver an RRC resume response to UE 402 in the MSG2 ofrandom access procedure. For UL small packet transmission, target cell406 may provide an acknowledgement (ACK)/non-acknowledgement (NACK)message in the MSG2 to indicate whether target cell 406 has received thesmall packet in MSG1 successfully.

FIG. 5 is a diagram illustrating a RRC resume procedure achieved by a4-step radio access procedure, according to an exemplary implementationof the present application. The wireless communication system 500includes UE 502 and target cell 506.

In step S510, UE 502 may send a preamble (e.g., a random accesspreamble) to target cell 506 through MSG1. In step S520, target cell 506may send a random access response in MSG2, if target cell 506 decodesMSG1 successfully. Target cell 506 may configure radio resource in MSG2for UE 502 to deliver MSG3. In step S530, UE 502 may deliver an RRCresume request in MSG3 to target cell 506. In some embodiments, for ULsmall packet transmission, the small data may be multiplexed with theRRC resume request in MSG 3. In step S540, target cell 506 may deliveran RRC resume response through MSG4 to UE 502. For UL small packettransmission, target cell 506 may provide an ACK/NACK message in theMSG4 to indicate whether target cell 506 has received the small packetin MSG3 successfully.

It should be noted that each of the 2-step and 4-step random accessprocedures may be applied to both the intra-RAT RRC resume and inter-RATRRC resume procedures.

FIG. 6 is a diagram illustrating an intra-RAT RRC resume procedure forRAN-based notification area (RNA) update, according to an exemplaryimplementation of the present application. In this embodiment, UE maysend an RRC resume request to a target cell for a RAN-based notificationarea (RNA) update.

In step S610, UE 302 may receive an RRC suspend message from source cell304 (e.g., a 5G-WB cell). In this embodiment, the RRC suspend messageincludes an RNA configuration for RNA update. The RNA configuration mayinclude at least one RNA ID or at least one physical cell ID.

In step S612, neighboring cells of source cell 304, such as target cell306, may broadcast physical cell ID or RNA ID. After checking thereceived physical cell ID or RNA ID with the stored RNA configuration,UE 302 may trigger RNA update. For example, UE 302 may trigger RNAupdate if the received physical cell ID/RNA ID is not found in thestored RNA configuration. In some embodiments, the RNA update istriggered periodically based on a given RNA update timer, which may beprovided in the RRC suspend message. When UE 302 is configured with anRNA update timer, UE 302 would start counting the RNA update timerduring RRC inactive state (or RRC idle state). UE would trigger RNAupdate progress (i.e., S622, S624, and S640) with RAN after the RNAupdate timer expires. Then, UE would reset the RNA update timer againafter the RNA update with RAN succeeds. The RNA update timer may bereleased after UE 302 transitions to other RRC states.

In step S622, UE 302 may establish connection with target cell 306 byusing random access procedure. In step S624, UE 302 may trigger RNAupdate by sending an RNA update request within the RRC resume request.Target cell 306 may identify source cell 304 of UE 302 after receivingthe RNA update request. Then, in step S630, target cell 306 may deliverthe RNA update request, which includes the physical cell ID of thetarget NR NB-IoT cell, to source cell 304. After receiving the RNAupdate request from target cell 306, source cell 304 may update the RNArecord of UE 302 in UE 302's context.

In step S632, source cell 304 may send an RNA update response to targetcell 306. Moreover, the target cell 306 and/or source cell 304's RAN mayprovide new RNA configuration to UE 302 in the RNA update response. Instep S640, target cell 306 may deliver an RNA resume response to UE 302.UE 302 may replace the stored RNA configuration with the new RNAconfiguration. It is worthy to note that, UE 302 does not undergo RRCstate transition during the RNA update procedure. In other words, UE 302stays in RRC inactive state or RRC idle state during the RNA update.

FIG. 7 is a diagram illustrating an intra-RAT RRC resume procedure forcore network (CN)-based tracking area update, according to an exemplaryimplementation of the present application. The wireless communicationsystem 400 includes UE 402, source cell 404, target cell 406, and CN408. In this embodiment, source cell 404 is a next generation wide bandcell, while target cell 406 is a NR NB-IoT cell, where source cell 404and target cell 406 are implemented under the same radio accesstechnology.

In step S710, UE 402 may receive an RRC suspend message from source cell404 (e.g., a 5G-WB cell). For example, source cell 404 may send the RRCsuspend message, when UE 402 or source cell 404 determines that theradio link quality between them is getting poor and/or satisfies one ormore predefined conditions. In this embodiment, the RRC suspend messageincludes tracking area information for CN-based tracking area update.The tracking area information may include at least one physical cell IDor at least one tracking area ID.

In step S712, target cell 406 may broadcast physical cell ID or trackingarea ID. UE 402 may decide to trigger CN-based tracking area update bychecking physical cell ID or tracking area ID received from target cell406 and the stored tracking area information. For example, UE 402 maytrigger CN-based tracking area update if the received physical cell IDor tracking area ID is not found in the stored tracking areainformation. In some other embodiments, the CN-based tracking areaupdate is triggered periodically based on a given TA update timer, whichmay be provided in the RRC suspend message. When UE402 is configuredwith a TA update timer, UE402 would start counting the TA update timerduring RRC idle state (or RRC inactive state). UE would trigger TAupdate progress (i.e., S722, S724, and S740) with network after countingTA update timer expires. Then, UE would reset the TA update timer afterthe TA update with network succeeds. The TA update timer may be releasedafter UE 402 transitions to other RRC states.

In step S722, UE 402 may establish connection with target cell 406 byusing random access procedure. In step S724, UE 402 may send a trackingarea update request within the RRC resume request to target cell 406. Instep S730, a tracking area update procedure is performed. In step S730,target cell 406 may forward the tracking area update request to sourcecell 404. Then, source cell 404, CN 408, or target cell 406 maycooperate to achieve tracking area update procedure for UE 402. UE 402'sUE context (and thus the RRC connection) may be switched to target cell406 after the tracking area update procedure. In step S740, target cell406 may deliver RRC resume response, which contains a tracking areaupdate response, to UE 402 after the tracking area update procedure isperformed successfully. The RRC resume response may include new trackingarea information. Thus, UE 402 may replace the stored tracking areainformation with the new tracking area information.

In one embodiment, the RRC resume request includes an RRC resume requestcause. For example, the RRC resume request cause may be a UL packettransmission, a DL packet reception (after the UE receives pagingmessage from RAN), a UL small packet transmission, an RNA update, or aCN-based tracking area update. It is noted that the UL small packettransmission may be a special case of UL packet transmission, whichhappens to an RRC inactive UE. RRC inactive UE may deliver a smallpacket without transitioning to RRC connected state (or temporarytransitioning to RRC connected state) and it might influence thebehaviors of the UE and RAN in the random access procedure. Thedefinition of small packet (e.g. upper bound of the size of smallpacket) may be pre-configured to the UE (e.g., through broadcastmessage) or be configured with the RRC suspend message.

In one embodiment, the RRC resume request includes source cellinformation. For example, the source cell information may includeeither: 1) a source RAT (e.g., NR NB-IoT, E-UTRA); or 2) a source PLMNID, which is obtained from the broadcasting message (e.g., systeminformation) of the source cell; or 3) a source cell ID.

Inter-RAT RRC resume procedure enables a UE to switch from a source cellto a target cell, where the source cell and target cell are implementedusing different radio access technologies.

FIG. 8 is a diagram illustrating an inter-RAT RRC resume procedure forpacket transmission/reception, according to an exemplary implementationof the present application. The wireless communication system 500includes UE 502, source cell 504, and target cell 506. In thisembodiment, source cell 504 is an E-UTRAN (Evolved Universal TerrestrialRadio Access Network) cell, while target cell 506 is a NR NB-IoT cell,where source cell 504 and target cell 506 are implemented usingdifferent radio access technologies. In some embodiments, the sourcecell is a legacy NB-IoT cell and the target cell is a NR NB-IoT cell of5G-RAN or vice versa. The source cell is an E-UTRAN cell and the targetcell is a 5G-WB cell or vice versa. In some embodiments, the source cellis a legacy NB-IoT cell and the target cell is a 5G-WB cell of 5G-RAN orvice versa. In some embodiments, the source cell is an E-UTRAN cell andthe target cell is a NR NB-IoT cell of 5G-RAN.

In step S810, UE 502 may receive RRC suspend message from source cell504 (e.g., E-UTRAN cell) to indicate UE 502 to implement inter-RAT RRCresume procedure. Source cell 504 may create and include an RRC resumeUE ID in the RRC suspend message to UE 502. UE 502 may transition fromRRC connected state to RRC idle state (or RRC inactive state) afterreceiving the RRC suspend message.

In step S822, UE 502 may attempt to find target cell 506, and performrandom access procedure with target cell 506 (e.g., a NR NB-IoT cell).In the inter-RAT RRC resume procedure, the RRC suspend message mayinclude the target RAT (e.g. NR NB-IoT), and/or target cell information.In some embodiments, source cell 504 may provide the target RAT and/ortarget cell information based on UE 502's measurement report about thesignaling qualities (e.g. RSRP, RSRQ, RSSI, SINR) UE 502 has measuredfrom one or more neighbor cells.

After successfully finding target cell 506, in step S824, UE 502 maysend the RRC resume request to target cell 506. The RRC resume requestmay include the RRC resume UE ID. After receiving the RRC resume requestfrom UE 502, in step S826, target cell 506 may attempt to obtain the UEcontext of UE 502 by sending UE context fetch request to source cell504. There are several approaches for target cell 506 to identify sourcecell 504. In one embodiment, target cell 506 may identify source cell504 by using the source cell ID included in the RRC resume UE ID. Inanother embodiment, UE 502 may add the source RAT (e.g. E-UTRA) and/orthe source cell ID, which is broadcasted by source cell 504 (e.g. in thesystem information of source cell 504), in the RRC resume request. Insome embodiments, target cell 506 may not have direct backhaulconnection with source cell 504. In such case, target cell 506 maydeliver UE context fetch request to a core network (not explicitly shownin FIG. 8). Then, the core network may help target cell 506 obtain theUE context of UE 502 by sending a UE context fetch request to sourcecell 504.

In step S828, source cell 504 may send a UE context fetch response totarget cell 506. The UE context fetch response may include the UEcontext of UE 502.

In step S830, target cell 506 may send RRC resume response to UE 502indicating the UE context of UE 502 is successfully obtained from sourcecell 504. In step S832, UE 502 may send an RRC resume complete messageto target cell 506. In step S834, after receiving the RRC resumecomplete message from UE 502, target cell 506 may send a UE contextrelease message to source cell 504 to let source cell 504 release orremove the UE context of UE 502. In step S836, source cell 504 mayoptionally forward the pending DL packets of UE 502 to target cell 506.

After the RRC resume complete message, UE 502 may transition to RRCconnected state from RRC inactive state (or RRC idle state), so that UE502 can establish a data connection with target cell 506 directly forpacket transmission and/or reception in step S840.

It should be noted that target cell 506 may require UE 502 to rebuild anew RRC connection by sending RRC resume response (new RRC connection)to UE 502. Then, UE 502 may clear the stored UE context and start RRCconnection establishment process with target cell 506. Also, target cell506 may reject the RRC Resume request by sending RRC resume responsewith a rejection message to UE 502. When UE 502 is rejected by targetcell 506, UE 502 may remove target cell 506 from the suitable cell(s)and find another cell for RRC Resume request.

For inter-RAT packet reception, after receiving the RRC suspend message,UE 502 may start packet reception if UE 502 receives paging message fromthe next generation radio access network (e.g., 5G-RAN). Then, UE 502may also start the radio access procedure and the following RRC resumeprocedure.

In some embodiments, an RRC resume validity timer is applied toinvalidate (part of) the information in UE context and/or RRC suspendmessage. FIG. 9 is a diagram illustrating a validity period for UEcontext storage at UE and source cell for inter-RAT/intra-RAT RRC resumeprocedure, according to an exemplary implementation of the presentapplication. The wireless communication system 900 includes UE 902 andsource cell 904.

For example, the RRC resume validity timer may be utilized to invalidateUE context at both the UE side and the network (NW) side. In step S910,source cell 904 may provide the RRC resume validity timer (UE side)value in the RRC suspend message to UE 902. After the RRC suspendmessage is sent to UE 902, source cell 904 may activate or start the RRCresume validity timer on NW side. Meanwhile, UE 902 may activate orstart the RRC resume validity timer on UE side after the RRC suspendmessage is received from source cell 904.

Source cell 904 may maintain the UE context of UE 902 before theexpiration of the RRC resume validity timer (NW side). UE 902 may find atarget cell and require for inter-RAT/intra-RAT RRC Resume before theexpiration of the RRC resume validity timer (UE side). However, it isalso possible that the UE may not find out the target cell before theRRC resume validity timer expires. In this condition, in step S920, UE902 would remove or clear the stored UE context after the expiration ofthe RRC resume validity timer on the UE side. In addition, source cell904 would remove the UE context of UE 902 after the expiration of theRRC resume validity timer on NW side. Therefore, after the expiration ofthe RRC resume validity timer, UE 902 may connect with RAN by requiringan RRC connection establishment rather than RRC resume request.

In another implementation, the RRC resume validity timer may be utilizedto invalidate part of UE context/RRC suspend message. For example, thesuggested CE level and target cell information in the RRC suspendmessage may become invalid after the RRC resume validity timer expiresin both UE side and NW side. In such condition, UE 902 may still applyRRC resume procedure with the remaining valid information in the UEcontext and RRC suspend message. Source cell 904 may indicate clearlywhich part in the UE context and RRC suspend message is to becomeinvalid after the RRC resume validity timer expires on both the UE sideand the NW side. Source cell 904 may still store the valid UE contextand control information configured in the RRC suspend message after theRRC resume validity timer expires.

As described above, several RRC connection resume methods are provided.According to the RRC connection resume method, when the UE determinesthat the radio link quality between itself and the serving source cellis getting poor and/or satisfies a predefined condition, the UE mayreceive the RRC suspend message from the source cell, and then performthe RRC resume procedure with the target cell in response to the RRCsuspend message.

The embodiments shown and described above are only examples. Even thoughnumerous characteristics and advantages of the present technology havebeen set forth in the foregoing description, together with details ofthe structure and function of the present disclosure, the disclosure isillustrative only, and changes may be made in the detail, including inmatters of shape, size and arrangement of the parts within theprinciples of the present disclosure up to, and including, the fullextent established by the broad general meaning of the terms used in theclaims.

What is claimed is:
 1. A radio resource control (RRC) connection resumemethod of a wireless communication system, comprising: receiving, by auser equipment (UE), an RRC suspend message from a first base station;performing, by the UE, an RRC resume procedure with a second basestation in response to the RRC suspend message; and receiving, by theUE, an RRC resume response from the second base station, wherein the RRCsuspend message comprises: a) target cell information, and b) targetradio access technology information comprising numerology information.2. The RRC connection resume method of claim 1, further comprising:transmitting, by the second base station, a UE context fetch request tothe first base station in response to the RRC resume procedure; andreceiving, by the second base station, a UE context fetch response fromthe first base station in response to the UE context fetch request;wherein the first base station and the second base station is realizedby different radio access numerologies, one of the first base stationand the second base station is a wide-band numerology and another one isa narrow-band-Internet-of-Things numerology.
 3. The RRC connectionresume method of claim 1, further comprising: transmitting, by thesecond base station, a UE context fetch request to the first basestation in response to the RRC resume procedure; and receiving, by thesecond base station, a UE context fetch response from the first basestation in response to the UE context fetch request; wherein the firstbase station and the second base station is realized by different radioaccess technologies, and one of the first base station and the secondbase station is a narrow-band-Internet-of-Things numerology.
 4. The RRCconnection resume method of claim 1, wherein the step of performing theRRC resume procedure with the second base station comprises:transmitting, by the UE, an RRC resume request to the second basestation; wherein the RRC resume request comprises at least one of: a) anRRC resume request cause; and b) source cell information.
 5. The RRCconnection resume method of claim 1, wherein the RRC resume responsecomprises at least one of: a) a data packet acknowledgement (ACK) orNon-acknowledgement (NACK); and b) an area update response.
 6. The RRCconnection resume method of claim 1, further comprising: transmitting,by the UE, an area update request to the second base station;performing, by the second base station, an area update procedure withother network entities when the area update request is received;receiving, by the UE, an area update response from the second basestation.
 7. The RRC connection resume method of claim 1, wherein the RRCsuspend message further comprises an RRC resume validity timer, and theRRC connection resume method further comprises: starting, by the UE, theRRC resume validity timer when the RRC resume validity timer in the RRCsuspend message is received; and removing, by the UE, at least a part ofUE context when the RRC resume validity timer expires.
 8. A userequipment (UE), comprising: a processor configured to: receive an RRCsuspend message from a first base station; perform an RRC resumeprocedure with a second base station in response to the RRC suspendmessage; and receive an RRC resume response from the second basestation, wherein the RRC suspend message comprises: a) target cellinformation, and b) target radio access technology informationcomprising numerology information.
 9. The UE of claim 8, wherein theprocessor is further configured to transmit an RRC resume request to thesecond base station, and the RRC resume request comprises at least oneof: a) an RRC resume request cause; and b) source cell information. 10.The UE of claim 8, wherein the RRC resume response comprises at leastone of: a) a data packet acknowledgement (ACK) or Non-acknowledgement(NACK); and b) an area update response.
 11. The UE of claim 8, whereinthe processor is further configured to: transmit an area update requestto the second base station; and receive an area update response from thesecond base station.
 12. The UE of claim 8, wherein the RRC suspendmessage further comprises an RRC resume validity timer, and theprocessor is further configured to: start the RRC resume validity timerwhen the RRC resume validity timer in the RRC suspend message isreceived; and remove at least a part of UE context when the RRC resumevalidity timer expires.
 13. A base station, comprising: a processorconfigured to: transmit an RRC suspend message to a user equipment (UE);receive a UE context fetch request from a second base station; andtransmit a UE context fetch response to the second base station inresponse to the UE context fetch request, wherein the RRC suspendmessage comprises: a) target cell information, and b) target radioaccess technology information comprising numerology information.
 14. Thebase station of claim 13, wherein the base station and the second basestation is realized by different radio access numerologies, one of thebase station and the second base station is a wide-band numerology andanother one is a narrow-band-Internet-of-Things numerology.
 15. The basestation of claim 13, wherein the base station and the second basestation is realized by different radio access technologies, and one ofthe base station and the second base station is anarrow-band-Internet-of-Things numerology.
 16. The base station of claim13, wherein the processor is further configured to: receive an areaupdate request from the second base station; and transmit an area updateresponse to the second base station in response to the area updaterequest.
 17. The base station of claim 13, wherein the RRC suspendmessage further comprises an RRC resume validity timer, and theprocessor is further configured to: start the RRC resume validity timerwhen the RRC resume validity timer in the RRC suspend message istransmitted; and remove at least a part of UE context when the RRCresume validity timer expires.